The present invention relates to a system and method for reducing the formation of nitrogen oxides by gas fuel burner assemblies during a combustion process. More particularly, the present invention relates to a system for mixing an inert gas, such as combustion product generated by the fuel burner assembly, with a fuel gas to dilute the fuel gas prior to introducing the fuel gas into the burner assembly.
In recent years, concern about air pollution has grown, especially in larger cities across the United States. Due to this growing concern about air pollution, governmental regulations have been enacted to control various types of air pollutants created during typical combustion processes.
Nitrogen oxides are one group of air pollutants which have been targeted by regulatory bodies for increasingly stringent controls. Nitrogen oxides, often referred to by the symbol NO.sub.x, include both nitric oxide (NO) and nitrogen dioxide (NO.sub.2). Nitric oxide and nitrogen dioxide are both often contained in the combustion product created during combustion processes. Upon release into the atmosphere, nitrogen oxides mix with water vapor to form nitrous and nitric acid. These acids can cause photochemical smog and acid rain.
Nitrogen oxides are formed during combustion of natural gas, fuel oils, and other fuels by oxidation of nitrogen contained in the combustion air and by oxidation of "fuel bound nitrogen" which is chemically attached to the fuel. As the intensity of the flame of a combustion device increases, the temperature of the flame also increases which in turn increases the amount of nitrogen in the combustion air that is oxidized during the combustion reaction.
In combustion processes which utilize fuel oil, fuel bound nitrogen can provide up to 60 percent of the resultant nitrogen oxides contained in the combustion product. The remaining 40 percent of the nitrogen oxides contained in combustion product from a fuel oil combustion process is formed by oxidation of nitrogen supplied as part of the combustion air.
In the case of gaseous fuels, such as natural gas or propane, nitrogen contained in the gas fuel is not generally chemically bound with the fuel. Instead, the nitrogen is physically mixed with the gaseous fuel. Therefore, the small portion of nitrogen oxides produced by fuel bound nitrogen in gaseous fuels has little or no affect on the final emission level of nitrogen oxides from combustion product of gaseous fuels. The main component of nitrogen oxides produced during combustion of gaseous fuels is caused by oxidation of nitrogen contained in the combustion air.
Because of its relatively pollution free combustion characteristics, natural gas is the preferred fossil fuel for boilers and heating equipment in many areas of the United States, especially in southern California where the use of fuel oils is now strictly limited. Sever pollution problems have led authorities in southern California to impose stringent limits on emissions of nitrogen oxides. Conventional forced draft combustion devices are not capable of meeting these stringent limitations. Although southern California has the most stringent emission limitations on the emission levels of nitrogen oxides at the present time, many of the metropolitan areas across the United States are expected to impose similar stringent emission limitations in the near future. It is therefore necessary to design a system to reduce production of nitrogen oxides by combustion devices to an acceptable level.
One object of the present invention is to provide a system and method to reduce combustion derived nitrogen oxides in conventional boilers, water heaters, or other heat exchange or combustion devices.
Another object of the present invention is to improve fuel-air mixing within the burner assembly firing head and combustion zone.
Yet another object of the present invention is to provide a system which can be easily added to existing conventional forced gas boilers or incorporated into new boilers at a reduced expense from Previously available systems and methods of reducing nitrogen oxides.
Still another object of the present invention is to increase the thermal efficiency of combustion devices.
A further object of the present invention to improve combustion homogeneity to reduce emissions of partially burned or unburned fuel from combustion devices.
According to the present invention, an apparatus and method is provided for use with a boiler having a burner assembly, a combustion chamber, a flue for exhausting combustion product created by the burner assembly from the combustion chamber, and fuel supply means for supplying a fuel gas to the burner assembly. The apparatus includes means for mixing the fuel gas with an inert gas prior to introducing the fuel gas into the burner assembly. The apparatus also includes means for introducing the mixture of fuel gas and inert gas from the mixing means into the burner assembly.
The mixing means is formed to include a first inlet, a second inlet, and an outlet. The apparatus further includes means for supplying inert gas to the first inlet of the mixing means, and means for coupling the fuel supply means to the second inlet of the mixing means.
In a preferred embodiment of the present invention, the apparatus comprises a flue gas recirculation system including a housing configured to define a mixing region to combine fuel gas and combustion product from the combustion chamber of the boiler prior to introducing the fuel gas into the burner assembly. The housing is formed to include a first inlet, a second inlet, and an outlet. A pipe interconnects the flue of the boiler to the first inlet of the housing. A blower is used to force a portion of the combustion product through the pipe from the flue to the first inlet. The system further includes means for coupling the fuel supply means to the second inlet of the housing, and means for coupling the outlet of the housing to the burner assembly.
The means for coupling the fuel supply means to the second inlet includes at least one shutoff valve and a pressure regulator to control the flow rate of fuel gas into the housing. The pipe for interconnecting the flue and the first inlet includes a valve to control the flow rate of combustion product through the pipe, and a pressure switch to monitor the pressure of the combustion product delivered to the housing. The pressure switch stops recirculation of the combustion product if the pressure drops below a predetermined level.
The means for coupling the outlet of the housing to the burner assembly includes a mixture pipe having a valve to control the flow rate of the gas mixture from the housing to a gas manifold of the burner assembly in response to varying low demands of the burner assembly. The gas manifold is configured to direct flow of the gas mixture directly into a firing head of the burner assembly. The mixture of fuel gas and combustion product entering the gas manifold includes at least 30 percent combustion product.
The housing or mixing region includes a manifold and a nozzle situated inside the manifold. The nozzle is connected to the first inlet and is situated inside the manifold so that inert gas or combustion product entering the manifold through the nozzle is directed into a mixture pipe interconnecting the outlet of the manifold and the burner assembly where the combustion product mixes with fuel gas entering the housing through the second inlet.
One feature of the present invention is the provision of mixing means configured to mix fuel gas with an inert gas to dilute the fuel gas prior to introducing the fuel gas into the burner assembly. Advantageously, by mixing the fuel gas with the inert gas, the fuel gas is diluted which slows down the combustion reaction of the fuel gas in the burner assembly, thereby reducing the amount of nitrogen oxides formed as a result of the combustion reaction. The reduction in the production of nitrogen oxides is achieved without adversely affecting combustion stability, efficiency, or emissions of other pollutants such as carbon monoxide.
Another advantage gained by mixing the inert gas and the fuel gas is that the bulk volume of the fuel gas mixture is increased which permits better mixing of the fuel gas with the combustion air in the burner assembly.
Yet another advantage of mixing inert gas or flue gas with the fuel gas is that the fuel gas is warmed by the addition of hot flue gas. The warmer fuel gas mixture is able to mix more easily with combustion air and to burn more homogeneously in conventional forced draft burners in which air-fuel mixing is generally uneven.
Still another advantage of mixing inert gas with fuel gas prior to introducing the mixture into the burner assembly is that the thermal efficiency of the boiler is improved by increasing the bulk volume of gases passing through the heat exchanger situated inside the combustion chamber. Part of the heat content of the recycled flue gas is returned to the combustion chamber which increases the thermal efficiency of the boiler.
Another feature of the present invention is that the system can be added to existing boilers or incorporated into new boilers at a much lesser expense than is possible with previously available systems. Advantageously, the present system can be added to a boiler without requiring major modifications of boiler design or adversely affecting the normal operating characteristics such as combustion stability, ignition reliability, and combustion efficiency of the boiler.
In this specification and in the claims, the word "boiler" is intended to refer to various types of combustion devices, such as boilers, water heaters, or other heat exchange devices, in connection with which the present invention may be used. The term "inert gas" refers to a gas or mixture of gases substantially comprising gas or gases which will not burn in the presence of oxygen.
Additional objects, features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.